Accumulating biochemical, histological, and genetic evidence supports the hypothesis that the 4 kDa β-amyloid protein (Aβ) is an essential component in the pathogenesis of Alzheimer's disease (AD). Selkoe D J (1997) Science 275:630-631; Hardy J (1997) Proc Natl Acad Sci USA 94:2095-2097. Despite the intense interest in the role of Aβ in the etiology of AD, the molecular mechanism of Aβ biosynthesis is poorly understood. The 39-43-residue Aβ is formed via the sequential cleavage of the integral membrane amyloid precursor protein (APP) by β- and γ-secretases. Selkoe D J (1994) Annu Rev Cell Biol 10:373-403. β-Secretase cleavage of APP occurs near the membrane, producing the soluble APPS-β and a 12 kDa C-terminal membrane-associated fragment (CTF). The latter is processed by γ-secretase, which cleaves within the transmembrane domain of the substrate to generate Aβ. An alternative proteolytic event carried out by α-secretase occurs within the Aβ portion of APP, releasing APPS-α, and subsequent processing of the resulting membrane-bound 10 kDa CTF by γ-secretase leads to the formation of a 3 kDa N-terminally truncated version of Aβ called p3.
Heterogeneous proteolysis of the 12 kDa CTF by γ-secretase generates primarily two C-terminal variants of Aβ, 40- and 42-amino acid versions (Aβ40 and Aβ42), and parallel processing of the 10 kDa CTF generates the corresponding C-terminal variants of p3. Although Aβ42 represents only about 10% of secreted Aβ, this longer and more hydrophobic variant is disproportionately present in the amyloid plaques observed post mortem in AD patients (Roher A E et al. (1993) Proc Natl Acad Sci USA 90:10836-40; Iwatsubo T et al. (1994) Neuron 13:45-53), consistent with in vitro studies illustrating the kinetic insolubility of Aβ42 vis-à-vis Aβ40. Jarrett J T et al. (1993) Biochemistry 32:4693-4697. Importantly, all genetic mutations associated with early-onset (<60 years) familial Alzheimer's disease (FAD) result in increased Aβ42 production. Selkoe D J (1997) Science 275:630-631; Hardy J (1997) Proc Natl Acad Sci USA 94:2095-2097. An understanding of the production of Aβ in general and that of Aβ42 in particular is essential for elucidating the molecular mechanism of AD pathogenesis and may also lead to the development of new chemotherapeutic agents which strike at the etiological heart of the disease.
Both γ-secretase and β-secretase are attractive targets for inhibitor design for the purpose of inhibiting production of Aβ. While γ-secretase is an attractive target for inhibitor design, little is known about the structure, mechanism, or binding requirements of this unidentified protease. Studies during the past few years suggest that γ-secretase is an unusual aspartyl protease with an intramembrane active site located within a multi-pass membrane protein called presenilin. Wolfe M S (2002) Nat Rev Drug Discov 1:859-866; Wolfe M S et al. (2002) Science 296:2156-2157.
In view of the foregoing, a need still exists to develop compositions and methods for treating disorders characterized by the production and deposition of β-amyloid.